Generally, an automatic transmission achieving more speed stages has been developed to enhancing fuel economy and optimizing drivability. Recently, increase of oil price is triggering a hard competition in enhancing fuel consumption of a vehicle.
Therefore, many researches for reducing weight and enhancing fuel economy through downsizing of an engine and for securing drivability and fuel economy through multiple speed stages of automatic transmissions have been developed.
However, in the automatic transmission, as the number of speed stages increase, the number of internal components (particularly, planetary gear sets) increase, and as a result, a length of the transmission increases. Therefore, mountability, cost, weight, transmission efficiency, and the like may still deteriorate.
Accordingly, development of a planetary gear train which may achieve maximum efficiency with a small number of components may be important in order to increase a fuel economy enhancement effect through the multiple-speeds.
In this aspect, in recent years, 8-speed automatic transmissions tend to be implemented and the research and development of a planetary gear train capable of implementing more speed stages has also been actively conducted.
However, a conventional 8-speed automatic transmission typically includes three to four planetary gear sets and five to seven control elements (frictional elements). In this case, since the length of the automatic transmission increases, mountability may be deteriorated.
Recently, one planetary gear set is disposed above another planetary gear set, but structures of automatic transmissions to which parallel planetary gear sets is applied are very limited.
In another way, dog clutches instead of control elements of wet-type are used. However, shift feel can be deteriorated.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.